1. Field of the Invention
The present invention relates to a laser including an angular dispersion element, and particularly to a laser including a beam expander for varying the magnitude of the angular dispersion provided by the dispersion element.
2. Discussion of the Related Art
Lasers emitting spectrally narrowed output beams are useful in a broad range of applications. Among these lasers, excimer lasers including KrF-, ArF-, XeCl-, XeF- and F2-lasers, exhibit output emission wavelengths in the deep ultraviolet (DUV) and the vacuum ultraviolet (VUV) regions of the electromagnetic spectrum. A typical setup of an excimer laser system includes a resonator and a discharge chamber filled with a gas mixture and connected to a power supply for generating an output beam, and a wavelength selection unit.
Without a wavelength selection unit, the natural output beam of an excimer laser is spectrally very broad (e.g., having a linewidth around 500 pm) compared with a linewidth desired for applications in microlithography (around one picometer or less). The linewidth is thus narrowed by the wavelength selection unit which also allows a particular narrow band of wavelengths within the broadband spectrum of the excimer laser to be selected as the output.
The wavelength selection unit typically includes one or more prisms and/or a grating or a highly reflective mirror producing angular wavelength dispersion. The dispersive nature of the prism and/or grating causes the linewidth of the output beam to be narrowed because only the rays within the acceptance angle of the resonator are selected as laser radiation evolves. Conventional wavelength selection units exhibit a fixed dispersion or beam expansion meaning that the dispersion or expansion ratio cannot be adjusted during laser operation. Consequently, to adjust the linewidth of a conventional laser, the system must be taken off-line and the wavelength selection unit adjusted or replaced by one that produces the now desired linewidth.
One prominent application of excimer lasers having narrowed spectral emission bands is microlithographic processing for creating various structures in microelectronics. Narrowed spectral linewidths are desired because minimum feature size and depth of focus are limited by chromatic aberrations of projection optics.
Dispersive gratings have been employed for spectral narrowing. See, e.g., U.S. Pat. No. 5,095,492 to Sandstrom; U.S. Pat. No. 4,696,012 to Harshaw. Prisms have been used as wavelength selection devices. See, e.g., M. Kakehata, et al., Experimental Study of Tunability of Discharge Pumped Molecular Fluorine Laser, Optical Society of America, Conference on Lasers and Electro-Optics (CLEO), Vol. 7 (May 1990). Fabry-Perot etalons have also been employed as wavelength selection devices. See M. Okada and S. leiri, Electronic Tuning of Dye Lasers by an Electro-Optic Birefringent Fabry-Perot Etalon, Optics Communications, vol. 14, No. 1 (May 1975). Birefringent plates have also been used for wavelength selection. See A. Bloom, Modes of a Laser Resonator Containing Tilted Birefringent Plates, Journal of the Optical Society of America, Vol. 64, No. 4 (April 1974); See also U.S. Pat. No. 3,868,592 to Yarborough et al. Unstable resonator configurations have been employed within pulsed excimer lasers. See, e.g., U.S. Pat. No. 5,684,822 to Partlo. U.S. Pat. No. 4,873,692 to Johnson et al. discloses a solid state laser including a rotatable grating and a fixed beam expander for narrowing the linewidth and tuning the wavelength of the laser. Further background information on methods of spectral linewidth narrowing of lasers can be found in textbooks on the tunable lasers. See, e.g., A. E. Siegman, Lasers (1986).
In addition, U.S. Pat. No. 4,972,429 to Herbst describes an achromatic beam expander for a high magnification and tunable laser. U.S. Pat. No. 5,596,456 to Luecke describes an achromatic anamorphic prism pair. These achromatic setups are designed to produce an output beam of a laser whose beam direction is independent of wavelength, and each includes an arrangement of prisms that are fixed in their orientations. Neither arrangement produces a beam wherein the linewidth and wavelength are adjustable.
The spectral linewidth of a laser output depends on several factors, including the angular dispersion of the wavelength selection unit, the angular acceptance of the resonator, and pulse duration and shape. In an excimer laser, since relatively few roundtrips occur per pulse, even small changes in pulse duration and/or shape can cause noticeable changes in the output emission linewidth. Any change in the angular acceptance of the resonator, which depends on such factors as the electrical discharge intensity distribution and the geometrical shape of the electrodes, may also cause the linewidth to vary. These factors are difficult to control. Moreover, minimal downtime for higher throughput is important in today""s industrial environment. Therefore, there is a need for a reliable way of automatically adjusting laser output emission linewidths xe2x80x9con-linexe2x80x9d, i.e., during operation of the laser. At the same time, it is also desired to have a laser system whose output emission wavelength and linewidth may be separately controlled.
It is an object of the invention to provide a laser system having a wavelength selection unit with variable dispersion such that the linewidth of the output laser beam is adjustable to a selected value.
It is a further object to provide the above laser system such that the wavelength of the output laser beam may be held to selected values when the dispersion is varied.
It is a further object to provide the above laser system such that each of the linewidth, wavelength are monitored and adjusted while the system is on-line using a feedback loop.
To meet these objects, a tunable laser system is provided including a resonator and a gain medium excited by a power supply for generating a spectral output emission, and a wavelength selection unit including an angular dispersion element. The wavelength selection unit also includes an adjustable beam expander for adjustably magnifying the angular dispersion. Separate optical elements, such as a grating and one or more prisms, or a single element, such as a prism, may provide the dispersion and the adjustable beam expansion. The adjustable beam expander preferably includes two synchronously rotatable prisms wherein a refraction angle change caused by rotation of one of the prisms is compensated by rotation of the other prism.
A processor preferably adjusts the orientation of the rotatable prisms and the tilt angle of a grating or highly reflective mirror of the resonator to control the linewidth and wavelength of the output laser beam. The linewidth may be controlled and/or adjusted to a certain linewidth on-line and automatically while the wavelength is maintained as desired by the processor in a feedback loop.